With recent growth in production of steel products by means of electric furnaces, much attention has been drawn to a technology of obtaining ferrous material as a feed therefor by solid reduction of iron ores. There has since been disclosed a process, representative of the technology, wherein solid metallized iron is produced by forming agglomerates, so-called "pellets", from fine iron ore in admixture with powdery solid reductants, and then reducing iron oxides contained in the fine iron ore through heating of the agglomerates at a high temperature (reference: for example, specification of U.S. Pat. No. 3,443,931, and Japanese Patent Laid-open No. 7-238307).
The process of reducing fine iron ore as disclosed in U.S. Pat. No. 3,443,931 described above comprises generally the following steps of:
1) forming green pellets by mixing fine iron ore with powdery solid reductants such as coal, coke, and the like, PA1 2) removing water adhered to the green pellets by heating same in such a temperature range that combustible volatile constituents issued therefrom are not ignited, PA1 3) reducing dried pellets by heating same at a high temperature to raise a metallization ratio, and PA1 4) cooling metallized pellets before discharging same out of a furnace. PA1 1) As an agglomerate (pellet) as merely agglomerated does not have sufficient physical strength to withstand handling during the process, it requires drying before charged into a reduction furnace. This entails installation of a drying unit in addition to a pelletizing furnace of complex construction, involving fairly high costs of operation and maintenance thereof. Furthermore, owing to a longer time required in the process from a step of drying the pellet to completion of a reduction step, production efficiency of the process is low, and it is difficult to hold down a cost of producing reduced iron. PA1 2) It is impossible to avoid generation of particles outside a predetermined size range during the pelletizing process. As it is necessary to recycle undersize particles to a mixing step, and to crush oversize particles before recycling to the mixing step, the production efficiency is poor. PA1 3) Iron oxides generated at steel mills such as iron-bearing dust, sludge, scale, and the like are among precious ferrous materials, however, these are often found in lumpy form when recovered, composed of fine particles bonded together, or in a form too large as pellet feed as in the case of scales. Accordingly, for pelletizing these iron oxides on their own in place of iron ore fine, or in admixture with iron ore fine, it is necessary to pulverize them to a predetermined size beforehand, necessitating installation of a pulverizing apparatus. PA1 (1) A method of producing reduced iron from fine iron oxides comprising the steps of a) through d) as follows; PA1 (2) A facility for carrying out the method (1) described above comprising; PA1 (3) A method of producing hot metal from fine iron oxides after completion of the steps a) through d) of the method (1) described above, comprising steps of e) through g) described below; PA1 (4) A method of producing hot metal from the fine iron oxides after completion of the steps a) through d) of the method (1) described above, comprising steps of e) through g) described below:
The conventional process of producing reduced iron as disclosed in U.S. Pat. No. 3,443,931 described above (for the sake of convenience, referred to as "pelletizing process" hereinafter) has fundamental problems as follows:
It is known that in reduction reaction of pellets, the higher a temperature at which the reaction takes place, the more rapidly the reaction proceeds. Hence, it is essential to heat up the pellets to a predetermined temperature rapidly by increasing a warming rate in order to improve productivity by increasing a reduction reaction rate. The process disclosed in the Japanese Patent Laid-open No. 7-238307 described above is characterized in that for a while after pellets are charged into a furnace, an oxygen containing gas is supplied onto the surface of the charged pellets, causing combustible matter issued from the pellets to be actively combusted so that a temperature on the surface of pellets is elevated to an optimum temperature for reduction by heat of combustion.
The process disclosed in the Japanese Patent Laid-open No. 7-238307 described above, however, belongs to a category of the pelletizing process consisting of steps of mixing, agglomeration, and drying, hardly solving the problems of the pelletizing process described above.
A furnace provided with a horizontally rotatable hearth (referred to as rotary hearth hereinafter) for heating is drawing attention in producing the reduced iron, and a same type furnace (referred to as rotary hearth furnace hereinafter) is used in the process as disclosed in the U.S. Pat. No. 3,443,931.
The rotary hearth furnace is characterized by its low capital cost as opposed to the case of a rotary-kiln furnace which has been in practical use over many years, however, due consideration should be given to charging of raw materials and discharging of a product since the hearth is horizontally rotated in the former case.
FIG. 1 is a schematic representation showing an example of conventional processes of producing the reduced iron by use of the rotary hearth furnace for heating of raw materials. As shown in the figure, fine iron ore 3 crushed to a predetermined size by a crusher 1, and pulverized coal 4 prepared by a dryer 2 and crusher 1 with bentonite 5 as binder added thereto are kneaded and mixed by a mixer 6 while water 7 and tar 8 are further added thereto. Mixed raw materials thus obtained are agglomerated by a pelletizer 9 or double-roll compactor 10, transferred to a feeder 12 of the rotary hearth furnace 11, and charged into the furnace, producing solid metallized iron by reducing iron oxides in the iron ore at a high temperature every time the rotary hearth 13 makes one turn. The metallized iron obtained is discharged from a product outlet 14. Reference numeral 15 denotes an exhaust outlet.
When the fine iron oxide and powdery solid reductants are kneaded and mixed after drying and crushing as necessary, a binder such as water, tar, theriac, organic resin, cement, slag, bentonite, quick lime, slightly burnt dolomite, or slaked lime is added thereto if need be.
The mixed raw materials are agglomerated into pellets in the shape of a ball by a desk pelletizer, or briquettes by the double-roll compactor. As the mixed raw materials of a particle size, 0.1 mm or less in diameter, are suitable for pelletizing, and same of a particle size, 1 mm or less, are for briquetting, the materials require prior pulverization to a predetermined size. In some cases, drying or curing treatment is applied to the agglomerates (that is, pellets and briquettes) to enhance physical strength thereof.
The agglomerates are sent to an upper part of the rotary hearth furnace via a belt conveyer, and charged via a feeding chute into the furnace so as to be spread in a wide area on the surface of the rotary hearth and smoothed out by a leveler. Subsequently, the agglomerates are heated and reduced while in rotation within the furnace, and turned into metallized iron.
The conventional process of producing reduced iron described above, however, has the following problem. That is, the agglomerates, due to powdering occurring thereto before charged into the rotary hearth furnace, will turn into agglomerates composed of particles of various diameters while generating fines, and charged onto the rotary hearth in such a condition. Then, generated fines are blown off by a combusting gas, and adhered in a molten condition to the wall of the furnace, causing troubles to facilities. In addition, the generated fines adhere in a molten condition to the rotary hearth, erode the hearth and roughen the surface of it.
Further, nonuniformity in firing results due to lack of uniformity in the size of the agglomerates, leading to the need of lengthening a firing time required for producing reduced iron of 92% metallization ratio, lowering productivity in producing the reduced iron.
Addition of the binder described above for prevention of an adverse effect of powdering of the agglomerates has been found effective to an extent, however, not successful in complete prevention of the powerdering. Furthermore, use of organic binders, which are expensive, results in a higher cost of production while use of inorganic binders having a constituent other than iron, that is, a slag constituent, has a drawback of degrading the quality of the reduced iron.
As described in the foregoing, the conventional pelletizing process has a number of problems.
Meanwhile, hot metal has been produced up to date primarily by the blast furnace process. In the blast furnace process, lumpy ferrous raw material and lumpy coke are charged into the furnace from the upper part thereof while hot blast is blown in through tuyeres provided in the lower part thereof so that the cokes are combusted, generating a reducing gas at a high temperature whereby iron oxides, main constituent of the ferrous material, are reduced and melted.
There has recently been developed another method of producing hot metal wherein reduced iron is produced by reducing lumpy ferrous raw material in a shaft reduction furnace, and the reduced iron in a hot condition is charged into a carbon material fluidized bed type melting furnace from the upper part thereof for reduction and melting. This method has already been put to practical application.
Various methods of producing hot metal directly from fine iron ores have also been developed. For example, in Japanese Patent Publication No. 3-60883, there has been disclosed a process wherein agglomerates are formed of fine iron ore and pulverized carbon material, the agglomerates are then prereduced in a rotary hearth furnace, and discharged at a temperature not less than 1000.degree. C. into a smelting furnace having a molten metal bath therein while the pulverized carbon material is fed under the surface of the molten bath, thereby reducing and melting the prereduced agglomerates in the smelting furnace. In this instance, off-gas discharged from the smelting furnace is recycled into the rotary hearth furnace for use as fuel for prereduction.
The conventional technologies described above, however, have drawbacks as follows:
Firstly, the blast furnace process has a drawback of requiring lumpy ferrous raw material and coke. In this process, cokes are formed in coke ovens through carbonization of coal, and only lumpy cokes are used after screening. Deposits of hard coking coal for use in making cokes are unevenly distributed in geographical terms. In addition, other major problems with the process are huge capital outlay required in replacing the old coke ovens, and needs for prevention of air pollution caused by operation of the coke ovens. With respect to ferrous raw material, fine iron ores need to be agglomerated into pellets or sinters for use in the process except the case where lumpy ores are used. In view of a tight supply position of lumpy iron ore, and high costs of pellets, however, use of sinters has come to be in the mainstream of the steel industry's practice in Japan, but countermeasures for prevention of air pollution caused by sintering operation poses a major problem to the industry.
In the process of producing hot metal in the shaft reduction furnace, coke is not required, however, the process has a problem of requiring lumpy iron ore as ferrous raw material as in the case of the blast furnace process.
A process described in Japanese Patent Publication No. 3-60883 is considered effective, however, has a drawback that fine iron oxides and powdery solid reductants need to be mixed and agglomerated before being charged into a reduction furnace.
In the course of agglomeration, particles outside a predetermined size range are inevitably generated as described hereinabove. Accordingly, undersize particles are sent straight to a mixing step while oversize particles need to be crushed before recycled to the mixing step, deteriorating efficiency of the process. In addition, since agglomerates as merely agglomerated do not have sufficient strength to withstand handling, the agglomerates need to be dried before charged into the reduction furnace, entailing installation of a drying unit in addition to an agglomeration plant. Costs of operation and maintenance thereof are also involved. All these factors add up the production cost of reduced iron. Furthermore, time required for agglomeration and drying is relatively long in comparison with that for reduction, adversely affecting the efficiency of a plant as a whole.
In the case of utilizing iron oxides generated at steel mills such as iron-bearing dust, sludge, scale, and the like, on their own or in combination with iron ores, these are often recovered in the form of "a lump composed of fine particles bonded together", or in "shape too large as pellet feeds" as in the case of mill scales. Accordingly, the iron oxides need to be pulverized beforehand to a predetermined size, necessitating installation of a pulverizing apparatus.
The present invention has been developed to provide a method of and a facility for production of reduced iron in a simple and inexpensive way in place of the conventional pelletizing method, and further, to provide a method of producing high quality hot metal efficiently and at a low cost through a simple process using reduced iron obtained as above.